Radiolabeled compounds are being developed for the purpose of tumor localization, and for following tumor growth and response to therapy. One area being explored is that of active-site directed gamma ray labeled enzyme inhibitors. Various substituents are being tried, both in side chains and in ring compounds. A second topic is the estimation of tumor growth and and regression, based on scan data (both scan defects as in liver scans, and cases in which tumors accumulate a radionuclide).